Size-Controllable Growth of Single Crystal In(OH)3 and In2O3 Nanocubes Tang,†
Zhou,†
Zhang,†
Qun Wenjia Wu Weichao Yu,† and Yitai Qian*,†,‡
Shaomin
Ou,†
Ke
Jiang,‡
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 147-150
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received March 8, 2004
ABSTRACT: Single crystal In(OH)3 nanocubes were synthesized by a designed novel hydrothermal treatment, and the size can be simply moderated by varying the hydrothermal temperature. By calcining the In(OH)3 nanocubes in air at 400 °C, single crystal In2O3 nanocubes were also prepared with the size slightly shrinking. Room temperature photoluminescence showed a broad photoluminescence emission spectrum in the blue-green region with its maximum intensity centered at 450 nm, which was mainly attributed to the effect of the oxygen deficiencies. Introduction Inorganic nanoparticles with well-defined shapes are of special interest to understand basic size-dependent, scaling laws and may be useful in a wide range of applications. Fields that would greatly benefit from advances in the synthesis of well-defined nanostructures include photonics, nanoelectronics, information storage, catalysis, and biosensors. Recently, a breakthrough was made in the synthesis of nanocubes. Monodispersed noble metal and Cu2O nanocubes were prepared by controlled oxidation-reduction reactions in surfactant solution and epitaxial electrodeposition on InP (001).1 Li et al. reported the fabrication of single crystal CaF2 nanocubes through a simple surfactant-free precipitation and hydrothermal procedure.2 Indium oxide and the closely related indium tin oxide are two important TCOs that are useful in wide applications such as electrooptic modulators, low emissivity windows, solar cells, flat panel displays, and electrochromic windows in dissipating static electricity from the windows on xerographic copiers.3 As In2O3 holds a wide gad gap (In2O3 direct gap of 3.55-3.75 eV) close to GaN, its nanostructure might hold possible applications in nanoscale optoelectronic devices. Furthermore, the possibility of using the nanostructure of In2O3 for UV lasers, detectors, and as gas sensors for ozone and nitrogen dioxide is appealing.4 Inorganic particles always showed unique size- and shape-dependent properties. For instance, the gas-sensing ability of In2O3 has been showed to increase significantly by decreasing its particle size.4 It is imaginable that size and shape controllable growth of In2O3 nanoparticles might pave the way to further elevate its performance. Recently, a In2O3 nanoparticle with a square or rhombohedral shape was subsequently produced.5 In this communication, we designed an oxidation hydrothermal route to In(OH)3 nanocubes in the absence of surfactant. In2O3 nanocubes were subsequently obtained by calcination of In(OH)3 nanocubes. As a wide band gap transparent * To whom correspondence should be addressed. Tel: 86-5513603204. Fax: 86-551-3607402. † Department of Chemistry. ‡ Structure Research Laboratory.
Figure 1. XRD pattern of the obtained In(OH)3 (a) and In2O3 (b) nanocubes.
semiconductor, the photoluminescence (PL) of In2O3 nanocubes was investigated in detail. Experimental Section The detailed experimental procedure is as follows: 35 mL of 4 M NaOH solution was fully mixed with 6 mL of 30% H2O2 in a 50 mL autoclave; subsequently, 2.0 g of metal indium particle was immersed into the solution, and then, the autoclave was sealed and put into an oven at 200 °C for 24 h. The white power was then collected from the solution, rinsed with deionized water, and dried in air at 40 °C. The as-prepared white powders were calcined in a boat crucible at a temperature of 400 °C and maintained for 2 h in air. The calcination temperature was determined by the DTA/TG (not shown here).
Results and Discussion The phase purity of the as-prepared hydrothermal and calcined products was evidenced with a Rigaku X-ray diffractometer with Cu KR radiation (λ ) 0.15418 nm). All of the reflection of the X-ray diffraction (XRD) pattern of the hydrothermal product in Figure 1a could be readily indexed to a pure body centered cubic phase [space group: Im3 (204)] of In(OH)3 with a lattice con-
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Figure 2. FESEM images of the In(OH)3 (a) and In2O3 (b) nanocubes.
Figure 3. TEM and SEM images of individual nanocubes: (a,d) TEM image of In(OH)3 and In2O3 square particles; (b,e) the corresponding ED patterns; and (c,f) SEM images of individual In(OH)3 and In2O3 nanocubes bound with crystal faces labeled.
stant of a ) 0.797 nm, compatible with the literature value of a ) 0.7979 nm (JCPDS 85-1338). The peak intensities in our samples follow those observed in the standard material except that our samples’ (420) reflections were a little more intense. Figure 1b shows that pure phase In2O3 can be obtained by calcination of the as-made In(OH)3. All of the peaks can be indexed to a pure cubic phase [space group: Ia3] of In2O3 with a lattice constant of a ) 1.002 nm, which is very consistent with the literature value of a ) 1.011 nm (JCPDS 71-2195). The overview morphology of In(OH)3 and In2O3 was observed by field emission scanning electron microscopy (FESEM), performed on a JEOL-JSM-6700F Scanning electron microanalyzer. The typical images of the In(OH)3 and In2O3 nanocubes are shown in Figure 2a,b. More than 95% of the particles of the as-obtained In(OH)3 were shaped with a regular cube with a mean edge length of 80-120 nm from 300 measured particles. The crystallinity of the as-synthesized In(OH)3 and In2O3 nanocubes was investigated by selected area electron diffractions (SAED) on a single particle. The square symmetrical SAED pattern recorded on the individual In(OH)3 nanoparticle (Figure 3a) could be attributed to [001] zone axis diffraction of bcc In(OH)3 (Figure 3b), which indicated that each particle was a single crystal. The SEM image of an individual square nanocube showed that the nanocube was heavily truncated and simply enclosed with crystal faces of {001} (Figure 3c). As the calcination products, In2O3 inherited its parents’ morphology indicated by FESEM (Figure 2b), but its size seemed smaller in that the latter
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dehydrated in calcination to cause a high density. The square structure of In2O3 was further examined with SAED recorded on a single nanocube (Figure 3d). The SAED pattern taken from the nanocube can be exactly indexed to a cubic In2O3 single crystal recorded from the [001] zone axis (Figure 3e). Thus, the cubes were so heavily bound with their {001} crystal faces (Figure 3f). Furthermore, no other In2O3 and In(OH)3 nanocubes with different truncated faces were found by SAED. In the previous years, sub- and micrometer In(OH)3 particles with different shapes and morphologies, from spherical to rodlike ones, have been produced by soft chemical methods, for example, by the sol-gel method, controlled precipitation in homogeneous solution, doublejet precipitation force hydrolysis, and sonohydrolysis.6 The original shape of the particles is kept in the phase transformation from In(OH)3 to In2O3 after calcination in the air. However, the well-shaped blocks of In(OH)3 and In2O3 are often composed of the smaller nanosized subunits; therefore, they have a polycrystalline nature. Alexander’s group synthesized cubic and hexagonal In(OH)3 and In2O3 microparticles, and the well-shaped blocks also have a polycrystalline nature composed of smaller irregular nanounits.7 In our designed procedure, single crystal cubic In(OH)3 was developed by hydrothermal treatment of the In-NaOH-H2O2 system. Our previous work was shown in the novel metal-alkalinehydrogen peroxide hydrothermal system. The obtained oxide showed abundant morphology and orientation.8 Furthermore, relevant properties with oxygen decencies displayed differences with bulk and other nanostructures. Excessive oxygen produced by H2O2 was accounted for the well-shaped and oriented nanoparticles, even for the novel optical properties, although we have not completely understood the role of H2O2. In the InNaOH-H2O2 hydrothermal condition, indium was oxidized into In(OH)3. The chemical equation was supposed to be expressed as:
In + H2O2 ) In(OH)3 + H2O Such a novel reaction is difficult to perform at room temperature, as indium can be stabilized in H2O2 solution even in the alkaline H2O2 solution. High temperature and atmosphere in the alkaline hydrothermal condition supplied a strong driving force to precede the reaction; the alkaline condition is necessary as it can stabilize the final product indium hydroxide. In such an In-NaOH-H2O2 hydrothermal system, In(OH)3 with the cubic phase crystallized with the cube shape. In principle, crystal growth and crystal morphology are governed by the degree of supersaturation, the diffusion of the reaction, the species to the surface of the crystals, the surface and interfacial energy, and the structure of the crystals; that is, extrinsic and intrinsic factors, the crystal structure, and the growth surroundings are accounted for in the final morphology. As we know, as for the different crystallographic faces, the growth rate is inversely proportional to the diffraction index; those crystallographic faces with low diffraction were always kept in the final products. As the final hydrothermal product, In(OH)3 was simply truncated with {001} faces as these faces hold the slowest growth rate. The cubical shape is consistent with the cubic crystal structure of In(OH)3. Another factor, which can also influence the
Single Crystal In(OH)3 and In2O3 Nanocubes
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Figure 4. FESEM images of the In(OH)3 cube obtained at different hydrothermal temperatures: (a) 150, (b) 200, and (c) 250 °C.
final morphology, is involved with the crystal growth surroundings. Plenty of oxygen produced by H2O2 in the hydrothermal condition is the special synthetic condition, which has scarcely been reported. The continuous feeding high content oxygen supplied enough nutrition to crystallize those faces in its original way. In conclusion, both the intrinsic structure and the favorable oxygen-rich condition gave rise to the formation of the In(OH)3 nanocube. Further study suggested that the size of the In(OH)3 nanocube strongly depended on the temperature in the hydrothermal treatment. A controlled experiment showed that the varying temperature led to a dramatic change in the size of the cubes when other synthetic parameters were fixed (Figure 4). When the temperature was moderated at 150 °C, the corresponding In(OH)3 cube owned the size of 50-80 nm, while at 250 °C, the size augmented into about 500 nm. A high hydrothermal treatment temperature is beneficial for the growth of the cube with a large size. However, the morphology and the single crystal nature remained except for a higher irregularity and wider distribution. In addition, controlled experiments carried out under various reactants’ concentrations (as for H2O2, from 2 to 9 mL and 2-7 M NaOH) showed that the concentration of reactant seems to have little impact on the size of In(OH)3 nanocubes. As the In(OH)3 nanocubes were calcined in the air, the morphology was inherited in the transformation from In(OH)3 to In2O3 with a slightly smaller size. The In2O3 nanocube also showed single crystal nature instead of a polycrystalline nature. The size of In(OH)3 can be modulated by the hydrothermal temperature. Correspondingly, as the calcination product, the size of In2O3 can be controlled by choosing an appropriate In(OH)3 precursor with a different size. The PL spectrum of the as-synthesized In2O3 nanocubes was measured on an F-850 fluorescence spectrophotometer (Hitachi) using an excitation wavelength of 380 nm at room temperature. It is known that the bulk In2O3 cannot emit light at room temperature;9 our experiment also showed that the commercial In2O3 power emitted no detectable light. However, the In2O3 nanostructure has been found to emit visible and ultraviolet light. Earlier reports indicated that In2O3 nanoparticles have PL peaks at 480 and 520 nm, In2O3 nanofibers exhibit strong and broad PL emission peaks centered at 470 nm,10 and In2O3 nanowires synthesized by the EDO method display strong PL emission peaks centered at 425, 429, 442, and 460 nm.11 Cao and coworkers even observed strong ultraviolet emission of In2O3 nanowires, which is attributed to the near band edge emission instead of the defect-induced emission.12 Figure 5 shows a strong and broad PL emission spectrum recorded from the present In2O3 nanocube with
Figure 5. Photoluminescent spectrum of the In2O3 nanocubes under an excitation wavelength of 380 nm with a Xe arc lamp at room temperature. The broad peaks are centered at 450 nm.
the size of 80-120 nm, which is mainly located in the blue-green region with its maximum intensity centered at 450 nm. As compared with the In2O3 nanoparticles reported in ref 10a, the main peak shifted to a higher energy; additionally, the shape of the spectrum is also beneficial for its application in optic devices although the intensity is slightly lower. The PL spectrum in the range of visible light was mainly attributed to the effect of the oxygen deficiencies. As for those wide gap oxides, semiconductors such as ZnO, SnO2, and In2O3, visible emission from oxygen deficiencies is easily detectable, and the deficiency concentration was always dependent on the growth surroundings. As for the In2O3 nanocubes, oxygen vacancies should be generated in the annealing process. Furthermore, the In2O3 nanocubes with a high aspect ratio and peculiar morphologies should also favor the existence of large quantities of oxygen vacancies. Similar to other oxide semiconductors, the oxygen vacancies would induce the formation of new energy levels in the band gap. As a result, the emission can be attributed to the radioactive recombination of a photoexcited hole with an electron occupying the oxygen vacancies, which is analogous to the PL mechanism of ZnO and SnO2 semiconductors. For deeper energy levels, the emission would shift to a lower energy level, and the blue-green light emission was also observed. Additionally, other kinds of defects such as indium vacancies or interstitials, stacking faults, and so on might also be introduced in the annealing process. Further work is in progress. Finally, the shape of the PL spectrum seems be independent of the particle size.
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Conclusions In summary, in the novel In-NaOH-H2O2 hydrothermal system, single crystal In(OH)3 nanocubes in high quality were synthesized successfully on a large scale. On the basis of SEM and SAED analyses, the cube was truncated with {001} faces as both intrinsic and extrinsic factors are favorable to the formation of nanocubes. Furthermore, its size can be varied by changing the hydrothermal temperature. By calcination, the In(OH)3 nanocubes transform into single crystal In2O3 nanocubes bound with {001} faces with a little smaller size; the PL emission, originating from oxygen vacancies, lies at the blue-green region with its maximum intensity centered at 450 nm. The as-obtained In2O3 nanocubes might perform better in optical and electric devices, solar cells, liquid crystal devices, UV lasers, detectors, and gas sensors because of its novel shape and controllable size. The synthetic and mechanism studies may supply the basis of novel nano/ microbuilding blocks and, more importantly, the success of bottom-up approaches toward further devices. Acknowledgment. Financial support from the National Nature Science Fund of China and the 973 Project of China are appreciated. References (1) (a) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (b) Gou, L. F.; Murphy, C. J. Nano. Lett. 2003, 3, 231. (c) Liu, R.; Oba, F.; Bohannan, E. W.; Ernst, F.; Switzer, A. Chem. Mater. 2003, 15, 4882.
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